OBJECTIVE: Using 4-dimensional computed tomographic scanner to determine the motion pattern of the acromioclavicular (AC) joint during adduction of the arm, with and without resisted superior elevation. METHODS: Sixteen healthy volunteers (5 women and 11 men; mean +/- SD age, 42 +/- 11 years). Four different motions were measured: AC joint width, anteroposterior translation, superoinferior translation, and opening of the superior aspect of the joint. Measurements between arm positions of neutral, adduction, and loaded were compared. RESULTS: Predominant movement is posterior translation (1.1 +/- 0.9 mm, P = 0.001); in the coronal plane, superior translation of the clavicle (0.6 +/- 0.5 mm, P = 0.001) and some opening of the superior joint space. Changes in the AC joint width and anteroposterior translation were significantly related to age (P = 0.016 and P = 0.006). CONCLUSIONS: Four-dimensional computed tomographic scans record the motion pattern of an asymptomatic AC joint and demonstrated that in adduction plus resisted elevation of the arm, the main movement of the AC joint is posterior and superior translation of the clavicle.